1. Field of the Invention
The present invention relates to devices for applying pressure to a web-like image-receiving material, and in particular to such a device for applying pressure to a continuous moving paper web in the transfer station of a non-mechanical printer or copier.
2. Description of the Prior Art
An image which is to be reproduced in a non-mechanical printer or copier such as, for example, a graphic image, is first generated on an intermediate image carrier in the printer or copier and in a printing or transfer station is then transferred to image-receiving material, such as a continuously moving paper web. The intermediate image carrier generally consists of a rotating roller having an outer surface which carries a toner image held in place by electrostatic charge. In order that the image may be transferred to the paper, the paper must contact the roller with no slippage between the roller surface and the paper web in the transfer zone of the copier so that the image is not smeared. The prevention of slippage is particularly crucial in the reproduction or printing of graphic images. During pauses between image transfers, the paper web is raised from the roller surface and remains stationary.
The intermediate image carrier may also be in the form of a foil which in the region of the printing zone passes about a roller for transferring the image to the paper, however, the principle of operation and the associated problems, are the same as described above. A device for pressing and lifting an image receiving material such as a paper web toward and away from an intermediate image carrier is described in German AS No. 2,903,265 which further comprises a transportation device for periodically transporting the paper web. This pressure device consists of a polarized solenoid having a movable pin which is connected to the armature of the solenoid and which is also attached to a pressure bar which extends along the entire width of the paper web at a right angle to the direction of transport of the paper web. The pressure bar has a generally C-shaped cross section. The pressure bar has two flank ends which can be bent outward or inward in order to assure a slight frictional resistance relative to the moving paper web without the need for special manufacturing of the flank ends.
In this known pressure device, however, residual frictional resistance which occurs during pressure-applying movement of the pressure bar produces an additional force component which in fact largely counteracts the force exerted by the transportation device, which may be operating simultaneously, on the paper web. This force component hinders transportation of the paper web.
A further problem with conventional pressure-applying devices is that when the paper web comes into contact with the surface of the intermediate image carrier at a relatively high speed the toner image, which is held on the intermediate carrier surface by electrostatic forces, will become partially atomized.